When a voltage is applied between a fine tip and a sample after the tip has been brought to within a few tens of a nanometer of the surface of the sample, a tunnel current flows. This tunnel current varies greatly, depending on the spacing between the tip and the sample, or the height of the tip above the sample. The distance between the sample and the tip can be quite accurately measured by measuring the amplitude of the tunnel current. A scanning tunneling microscope (STM) images the topography of the surface of a sample accurately, utilizing this phenomenon. For this imaging, the sample surface is scanned in two dimensions by the STM probe tip. During the scan, the distance between the tip and the sample, or the height of the tip above the sample, is controlled so that the tunnel current may be maintained constant. A signal which is used for this control is also employed to image the topography of the sample surface. This scanning tunneling microscope of this structure has attracted attention to analyze the arrangement of atoms at the sample surface. Also, the scanning tunneling microscope has rapidly spread to various applications in these years, because it is simple in structure and small in size. For example, the instrument is applied to measurement of surface roughness of semiconductor devices. This kind of scanning tunneling microscope is disclosed, for example, in U.S. Pat. No. 4,343,993.
Three-dimensional mechanical scanners of this construction used in scanning tunneling microscopes are classified into three major types. One type is shown in FIG. 5 and comprises three piezoelectric transducers 1X, 1Y, and 1Z which are designed to expand. These transducers are so assembled that they extend perpendicular to each other. This type of scanner is disclosed in the Physical Review Letters, Vol. 49, No. 1, 1982, published by the American Physical Society. Electrodes 2X.sub.1, 2X.sub.2, (2X.sub.2 is not shown), 2Y.sub.1, 2Y.sub.2, (2Y.sub.2 is not shown), and 2Z.sub.1, 2Z.sub.2 (2Z.sub.2 is not shown) are attached to the piezoelectric transducers 1X, 1Y, 1Z, respectively. Appropriate voltages are applied between the electrodes to move a probe tip 3 quite short distances in three dimensions. This type of scanner needs a large space, since the three piezoelectric transducers must be assembled so as to extend in mutually orthogonal directions as shown. Also, the piezoelectric transducer for scan in the x-direction interferes with the piezoelectric transducer for scan in the y-direction, because the three transducers are mounted so as to extend perpendicular to each other. As a result, the obtained STM image is distorted.
Another type of scanner is shown in FIG. 6, where an electrode 4Z for scan in the z-direction, electrodes 4X.sub.1, 4X.sub.2 (4X.sub.2 is not shown) for scan in the x-direction, electrodes 4Y.sub.1, 4Y.sub.2 (4Y.sub.2 is not shown) for scan in the y-direction are mounted to the outer surface of a piezoelectric transducer 4 which expands and consists of a hollow cylinder. A common electrode 4G is mounted on the inner surface of the transducer 4. This type is disclosed in Rev. Sci. Instrum., 57(8), August 1986, published by the American Institute of Physics. Appropriate voltages are applied between the common electrode and the other electrodes to move a probe tip 5 in three dimensions over quite minute distances. If a long piezoelectric transducer is used as the transducer 4 to extend the ranges of x and y scans, a distortion occurs at the ends in the z-direction. The amount of distortion produced is in proportion to the length of the piezoelectric transducer. As an example, if the transducer if 20 mm long, and if the probe tip is scanned over a distance of 2 .mu.m, then a distortion of about 400.ANG.is produced.
A third type is shown in FIG. 7 and comprises piezoelectric transducers 6X, 6Y, 6Z stacked on top of each other. The transducers 6X and 6Y cause shear deformations. The transducer 6Z causes an elongation as viewed from a probe tip 7. This type is disclosed in the Japanese Journal of Applied Physics, Vol. 27, No. 1, January 1988. Electrodes are attached to both faces of each piezoelectric transducer. Suitable voltages are applied to these electrodes to move the probe tip 7 over very short distances in three dimensions. In order to extend the ranges of x and y scans, it is necessary that numerous piezoelectric transducers be stacked. This complicates the wiring between the piezoelectric transducers and increases the cost.